Compound transistor pairs consisting of a bipolar transistor and a field effect transistor in a cascade arrangement are well known. They seek to combine the speed of a field effect transistor with the voltage handling capacity of a bipolar transistor. It has also been known to connect a zener diode, a voltage reference device, in parallel between the base of the bipolar transistor and the source of the field effect transistor for improving the switching properties of the transistor pair.
However, because the ordinary zener diode tends to have a high working resistance when designed to have a low zener voltage, this has not proved completely satisfactory. Alternatively, instead of a single zener diode, there has been employed in the same fashion a series of forward-biased diodes. However, such an arrangement suffers from the fact that the breakdown is soft rather than sharp and as a result the losses cannot be kept small.
The present invention seeks to improve the switching properties of a compound transistor pair by combining it with circuitry which essentially provides the ideal zener characteristic where needed.
Additionally in a preferred embodiment, the invention provides an arrangement which facilitates limiting the maximum current which can flow through the compound transistor pair.